Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting and signaling applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output.
Efficient high-power LEDs are often based on blue light emitting InGaN materials. To produce an LED based illumination device having a desired color (e.g. white) output, a suitable wavelength converting material, commonly known as a phosphor, may be provided which converts part of the light emitted by the LED into light of longer wavelengths so as to generate a combination of light having the desired spectral characteristics. An example of a suitable wavelength converting material for use in a blue LED based device for emitting white light is a cerium-doped yttrium aluminum garnet (YAG:Ce).
A disadvantage of the LED-phosphor based illumination devices is that in the functional off-state of the device, the color of the phosphor may be clearly visible. For example, YAG:Ce has a distinct yellowish appearance. Since in many applications a colored appearance of a light source in the off-state is undesired, techniques have been developed to produce LED based illumination devices having a neutral, e.g. white or whitish, appearance in the off-state. One such technique is disclosed in WO 2008/044171, which describes a lighting device comprising an LED and a phosphor layer. The lighting device further comprises means for providing a residual current to the LED in the functional off-state. As a result of the residual current running through the LED in the functional off-state of the lighting device, the tiny amount of light produced by the LED outshines the reflected ambient light, and hence the LED will appear to have a neutral color to the human eye.
However, there is a need in the art for improved light emitting devices, which in the functional off-state have a white or neutral appearance.